Is it normal for li ion batteries to heat up during use?

I assume it is, but being in a flashlight you don’t notice it, but i used a ML 101 to charge my cell phone for the first time and noticed the battery heated up, not hot but warm.

From my experiences warming under high load or rapid charging is common, however in some cases it can be a indication of a weak cell with high internal resistance. I’ve noticed that my old laptop pack pulls heat up a lot more under heavy use than my pulls from brand new packs.

I’m guessing that in your case its just that the ml101 puts out a bit more power than a normal phone charger would. I wouldn’t be too concerned about a few degrees.

I’m pretty sure the batteries in my Roche F8, or Trustfire X9 are warm when I take them out because they’re right under 3A drivers and the body tubes get nice and warm when running on high.

My batteries don’t get warm when in my i4. The i4 gets warm if all four slots are plugged, but the batteries themselves no.

Yes it is normal for any battery to warm up during discharge/charging. Li-Ion can go up to around 60C i believe, though Lithium ion cells generally perform better and provide a higher capacity output if they are warm. Around 20-30C is the ideal temperature for my preference. Look at this:
Panasonic states that discharging at 40C gives a higher capacity.

little warm ok.
excess heat may indicate high charging current than rated for safe charging.

some phones such as old nokia & se charging current is limited by charger.

I have sonyericsson j10i2 phone. its stock charger is 500mA max. charge phone without noticeable heat.
If I connectcharger that comes with SonyEricsson 850 which is 700/800mA. phone heat up

I think newer phones with usb charging doesnt have such issues.

it’s normal for any battery to ware up during discharge and charging.
the heat is due to the charging current.

I guess not everyone understood, i am talking about battery heating during discharge

My answer referred to both discharging and charging. They both heat up the battery which is normal. As long as it doesn’t get very hot 40C+ then should be fine.

high internal resistance ?

At a 1A load, most cells should be still be at room temperature. I have the ML102 and when I use it to charge my phone, the cell stays cool.

Weak cell.

Are they protected? I think some protection circuits can add resistance that causes the cell to warm. I had some Sanyos that kept getting hotter and hotter, making me nervous. I removed the protection, and now they are as cool as a cucumber.

Have you checked the boost circuit temperature? I guess it is warming the cell.

How old is the battery bort and how much use has it seen? I assume since its a cell phone its charged almost every day?

Hmm… can you measure the current draw?

18650’s shouldn’t be getting hot even when under a 2-3A load, unless the circuitry is heating it.

At 1A charge/discharge, all of my good cells sit two degrees C above ambient (cell in open air, not seated in a charger).
I find that some heat comes from the ML102, but it’s not a lot. I have had a cell or two exhibit higher than expected warm up during use and charge. Later I determined they had higher internal resistance than their siblings. More noticeable when charging, I find.
Once I see this, I charge on a power supply with leads to rule out heat from the charger. If it still heats up, marked as bad and recycled. Li-Ion cells are too cheap to take chances with, IMO.

Oh dear, just order a few cheap chargers rated 1A for single cell. Is it too high for single cell? Cos I can the rest was like 500mAh.

My cells which are LI-ON, during discharge on high on my lights they get HOT, on lower discharge they don’t heat at all, or not enough to notice, they get slightly warm when charged also.

Bort, all my old, new, protected, laptop pulls, etc 18650 heat up charging the phone with the ml102, the closer the cell is to be empty, the hotter it gets. I suppose it is because the current increases as the voltage drops.
they are only a bit warm, nothing uncomfortable to hold. perfectly normal for me